PVC as radome

Tue Aug 24, 2010 4:56 pm   

Hello,

I am attempting to use a schedule 40, grey PVC pipe as an antenna radome
and am concerned about RF losses.

My application is a UHF transmitter with an output power of 40 Watts (4
dBm) into an omnidirectional antenna with 3 dB gain, or total estimate
EIRP of 49 dBm (80 Watts). RF is Right Hand Circularly Polarized (RHCP
emissions at an operating frequency of 401 MHz.

I cannot tolerate more than about a half dB loss through the PVC pipe. Ar
there any significant RF losses in radiating such a signal through Schedul
40, grey PVC pipe?



---------------------------------------
Posted through http://www.Electronics-Related.com

Tue Aug 24, 2010 5:39 pm   

In article <zbOdneA13P8odu7R4p2dnAA_at_giganews.com>, "blanker" <shamale_at_n_o_s_p_a_m.hotmail.com> wrote:

What if you put it in a microwave oven and see if it heats up ?
Along with a load.

greg

Tue Aug 24, 2010 5:59 pm   

On 08/24/2010 08:56 AM, blanker wrote:

I know that it's a fairly popular material for amateur radio antennas at
HF, but I'm not sure about its properties at UHF.

Try your antenna with and without the PVC. Pay attention to de-tuning,
if any -- that would muck up calculations.

To get a quick estimate of the attenuation, you could wind a small coil
around some PVC and resonate it with a cap, then measure the Q. If I
did a measurement with and a measurement without the PVC core I would
expect a small but possibly significant change in resonant frequency,
and I would expect that after I finished measuring the Q change I'd have
a better notion of how well the pipe would work.

If you're going to do this for production, you'd be smart to set up a
test range and check every pipe, or at least every lot of pipe, before
you use it. Stuff like that can vary in how it's manufactured as long
as it does the job for which it's intended and marketed -- and radomes
isn't one of those jobs.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tue Aug 24, 2010 6:22 pm   

blanker wrote:


There is no quick answer to this question. A lot depends on the geometry
of your antenna and the particular spots with the high intensity of EM
field. The practical way would be try and see for yourself.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Tue Aug 24, 2010 6:23 pm   

mpm wrote:


RF is used for welding PVC, which has a dielectric constant
of about 4.5. Other common thermoplastics are downwards of a
half of that, so they can't be welded in the same way.

Ian

Tue Aug 24, 2010 6:31 pm   

On 08/24/2010 10:23 AM, Ian Iveson wrote:

Dielectric constant and absorption constant are two different things,
though. In principal there's no reason that a high dielectric material
can't have low absorption.

And again, there are so many modifiers that you can put into plastics
that you can't say "will plastic XYZ work as a radome". You have to ask
"can plastic XYZ _ever_ work in a radome, and how do I make sure to get
a flavor of it that will?".

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tue Aug 24, 2010 6:32 pm   

On 08/24/2010 10:22 AM, Vladimir Vassilevsky wrote:

Perhaps a quick "no" if the stuff is really badly absorptive.

But a "yes" certainly requires the experiment (and IMHO, constant repeat
testing if you're buying pipe meant for plumbing).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tue Aug 24, 2010 7:31 pm   

On Aug 24, 10:56 am, "blanker" <shamale_at_n_o_s_p_a_m.hotmail.com>
wrote:

Research the dielectric constant of PVC.
I expect it will be low, and reasonably close to air. (?)

Fiberglass and (dry) balsa wood are also reasonable candidates.
Cellular companies often use specialized structural composites (I
think it's called "FRX", or something like that.
Perhaps you can locate a source for these?
The materials used for microwave radomes (which I used to know the
name, but it escapes me at the moment), is another avenue to try.

IIRC, Maxrad antennas had some land-mobile antennas enclosed in PVC,
so you're probably fine.
Those would have been little UHF or 800 MHz antennas (5/8th over a 1/2
wave, or similar).
The PVC was thin-wall however, as you would expect in a mobile.

Tue Aug 24, 2010 7:45 pm   

PVC does introduce some loss and some de-tuning. A lot of hams use it
(e.g. for radomes on simple twin-lead J-pole antennas), but it's
difficult to predict whether any specific batch will have a low enough
loss to meet your requirements. Since it isn't being manufactured with
RF characteristics in mind, different batches or brands may be
significantly different in their RF behavior.

The suggestion to stick a few inches in a microwave (with a cup of
water some distance away), zap it for a few seconds, and see if it
heats up is a good one. It's not a certain test, of course, but if
the tube does warm up appreciably when exposed to microwaves, it's
probably not a good choice at UHF.

How large a diameter to you require? If it's anywhere up to 2.5" OD,
you might want to get some fiberglass tubing from Max-Gain Systems...
fiberglass-and-resin is a traditional RF radome material and it may have
lower losses than PVC.

If this is a one-off project, you might want to consider making
your own radome, using fiberglass cloth and resin... you could
probably fabricate a radome which has a thinner wall than most
commercial fiberglass or PVC pipe.

--
Dave Platt <dplatt_at_radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Tue Aug 24, 2010 7:47 pm   

On 08/24/2010 11:37 AM, mpm wrote:

Not necessary, although helpful.


Given the dielectric constant it _would_ change the tuning. That's not
the issue -- the issue is whether it absorbs the waves as well as
diffracting them.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tue Aug 24, 2010 8:50 pm   

In article <2e620bf0-cbb3-4cd6-8f38-4ca38f021bf8_at_d8g2000yqf.googlegroups.com>, mpm <mpmillard_at_aol.com> wrote:


I just thought of a grid dip meter !!

Tue Aug 24, 2010 9:37 pm   

On Aug 24, 11:39 am, zekfr...@zekfrivolous.com (GregS) wrote:

That's an interesting thought....!!
I would make sure the microwave oven has a turntable, or at least an
RF stirrer, as I'm sure most do nowadays?

I think you could also shred an amount of the PVC material and place
it into a tuned cavitity filter (like a big VHF or VHF bandpass cavity
filter) and check the response both before and after insertion of the
material.
If the tuning doesn't change too much, I would think the material
would be acceptable as a radome.

That said, I would not fill the cavity completely, or even allow the
PVC material to come into contact with the center plunger of the
cavity.
I think that may skew the results of the tests, but I would have to
think hard about the exact reason why....
It seems to me I did this test once before in my life... ????

-mpm

Tue Aug 24, 2010 10:18 pm   

GregS wrote:


Rather useless experiment. Whatever the result would be, it is largely
irrelevant if the material could be used for OP's antenna or not.

FWIW, once long ago I did a mistake: isolated a junction of coax cables
with PVC electric tape. The power level was ~1kW at ~60MHz. The tape
burned away in few hours. Then I used polyethylene for isolation and it
worked fine.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Wed Aug 25, 2010 2:08 am   

On 24 ago, 17:56, "blanker" <shamale_at_n_o_s_p_a_m.hotmail.com> wrote:

Hello,

It depends strongly on how close the radome is to the actual antenna.
When it is significantly within 0.25lambda of the antenna structure,
it will experience the (strong) reactive field. So you need to guess
(measure, simulate) the field strength experienced by the PVC.

With 40W, and about 80mm from the antenna, you may get about 1kV/m,
the worst case is when this field is parallel to a thin sheet of PVC
as the field inside the PVC equals the air field strength. Assuming Er
= 3 and tandelta = 0.015 gives about 1kW/m^3 material loss. With a
radome material thickness of 3mm, this would result in 3 W/m^2 loss
(guess surface area exposed by the field to find the "actual" loss).
When the unobstructed field is perpendicular to the material, loss
drops significantly (as the E-field inside the PVC drops to about 33 %
of the value in air).

0.5 dB from 40W is about 4W. When you increase the distance between
the antenna and the radome, E drops and loss/m^3 is proportional to
E^2. However if the material is very close to the antenna, over a
large area, overall loss may be above 0.5 dB. Considering PVC is not
that bad. Other issue to check: UV resistance and plastic deformation
(creep) under mechanical loading in combination with high
temperature.

I hope this gives you some guidance in whether it is worth to try
PVC.

Best regards,

Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me very likely

Wed Aug 25, 2010 2:32 am   

On Aug 24, 9:59 am, Tim Wescott <t...@seemywebsite.com> wrote:


Exactly.
Pure PVC is white, not gray; that color is a pigment of some sort, and
the manufacturer of 'schedule 40' pipe is making his tests on burst
strength, not RF absorption. It wouldn't be unlikely for the
labeling
ink on the pipe to be active in this case, either (hematite is a
common
pigment).